The aim of this research program is to understand in molecular detail the mechanism of DNA replication in a model eukaryote, Drosophila melanogaster. The investigation will be organized along the following lines. 1. DNA chain elongation: a. Dissociation and reconstitution of Drosophila polymerase-primase. (1) The 182 and 73 kDa subunits (2) The 60 and 50 kDa primase subunits (3) A cryptic 31-51 exonuclease in mammalian and yeast polymerase-primase (4) Use of Drosophila 182 kDa subunit to identify Drosophila single-stranded DNA binding protein (5) Replication of chromatin by polymerase-primase b.DNA polymerase 6 in Drosophila embryos 2. Initiation of DNA replication at a Drosophila origin in vitro a. Identification of a chorion origin binding protein(s) b. Initiation in vitro of chorion DNA replication 3. Regulation of DNA replication a. Changes in polymerase-primase during embryogenesis